The present invention generally relates to an apparatus for use in semiconductor wafer sawing, and more particularly to a holding unit for supporting a crystal ingot during a sawing process.
In semiconductor manufacturing, wafers are generally formed from grown crystal ingots. An ingot is a crystal formation grown from a seed in a melt under suitable temperature and pressure conditions. After suitable growth occurs on the seed, the resulting ingot is pulled-up in a conventional pulling apparatus from the melt. Next, a wafer may be sliced from the ingot using a cutting apparatus such as a saw.
Generally, an internal diameter saw includes an annular blade which has an inner edge coated with a hard material such as diamond powder or grit. Typically, the blade is rotated at a high speed as the ingot is advanced toward the blade. Alternatively, the inner edge of the blade may be moved through a stationary ingot. The inner edge of the blade contacts the ingot in a direction normal to the axis of the ingot to cut therethrough.
In many instances, a holding unit is attached to a lower side of the ingot to support the ingot during the sawing process. One such process is plunge mode. In this process, the saw is advanced completely through the ingot and the holding unit to form a wafer. The saw blade is then retracted to its initial position for the next saw cut.
In another sawing process, the saw may be advanced through the ingot, but only partially through the holding unit. This process may be repeated to yield a group of wafers that are manually separated from the holding unit.
When the holding unit is formed from a material which is different from the crystal ingot, the material from the ingot can tend to chip. The chipping of the ingot causes an increase in labor and material costs as well as a loss in yield.
For example, when the holding unit is formed from a more fragile material than the ingot, the saw cutting into the holding unit induces blade vibration. As a result, the contact pressure surrounding the blade is unstable which leads to chipping. The blade is also able to move laterally in the softer material. This tends to increase the stress on the material during its withdrawal from the ingot, and thus, increases the amount of chipping of the resulting wafer.
Another disadvantage in known sawing systems is that the saw blade tends to lose its shape because of the variance in materials of the ingot and holding unit. A densified powder layer may also form on the blade. As a result, the blade may be dulled. The resulting sliced wafer may be warped or bow in one direction.
In general, the present invention is directed towards a holding unit for supporting an ingot in a semiconductor wafer sawing machine. The holding unit is formed from substantially the same material as the ingot and includes a plurality of break points.
Accordingly, in one aspect, the invention is directed to a holding unit for use in a semiconductor ingot sawing machine that includes a top surface and a bottom surface. The top surface includes holding surfaces to secure an ingot of semiconductor material. First and second legs may extend from the bottom surface, and a cavity may be formed from the first and second legs and the bottom surface to form a plurality of break points in the holding unit. The holding unit and the ingot may be formed from substantially the same material.
Implementations of the invention include one or more of the following. An end of each of the first and second legs may be tapered. The holding unit and the ingot may be formed from silicon. The sawing machine may include a slide unit to move the holding unit with the ingot mounted thereto to a first position. An arm may be used to move a blade in a direction normal to the ingot when the ingot is in the first position. The blade may slice through the ingot and at least a portion of the mounting strip to form a wafer. The blade may be an internal diameter sawing blade.
In accordance with another aspect, the invention is directed to a holding unit for a semiconductor wafer sawing machine that includes a top surface and a bottom surface. The top surface may have a shape suitable to secure an ingot. The holding unit may also include a first and second cavity, and a plurality of break points may be formed from the cavities. The ingot and the holding unit may be formed from substantially the same material.